1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, it relates to an active matrix liquid crystal display device of a so-called in-plane switching type.
2. Description of the Related Art
A liquid crystal display realizes display in such a manner that an electric field is applied to liquid crystal molecules in a liquid crystal layer sandwiched by a pair of substrates to change the orientation direction of the liquid crystal, so as to cause optical change of the liquid crystal layer.
The conventional active matrix liquid crystal display device is represented by a twisted nematic (TN) display system, in which the direction of the application of the electric field to the liquid crystal is set in the direction perpendicular to the substrate plane that sandwiches the liquid crystal, and the display is realized by utilizing the optical rotation of the liquid crystal layer.
On the other hand, a liquid crystal display device of a in-plane switching (IPS) system has been proposed in JP-B-63-21907, U.S. Pat. No. 4,345,249, WO 91/10936 and JP-A-6-160878, in which a comb electrode is used, and the direction of the electric field applied to the liquid crystal is set in the direction parallel to the substrate plane, whereby the display is realized by utilizing the birefringence of the liquid crystal.
The in-plane switching system has advantages, such as a wide viewing angle and a low load capacitance, in comparison to the conventional TN system, and is being rapidly developed in recent years as a new active matrix liquid crystal display device that superseding the TN system.
In the IPS system, the in-plane switching can be more perfectly realized in the case where the liquid crystal has negative dielectric anisotropy in comparison to the case of a liquid crystal having positive dielectric anisotropy, as demonstrated in J. of Appl. Phys., vol. 82, No. 4, pp. 528-535 (1997) by M. Oh-e, M. Yoneya and K. Kondo. The liquid crystal having negative dielectric anisotropy has a dielectric constant in the short axis direction of the liquid crystal molecule that is larger than the dielectric constant in the long axis direction perpendicular thereto, and the liquid crystal having positive dielectric anisotropy has a dielectric constant in the short axis direction of the liquid crystal molecule that is smaller than the dielectric constant in the long axis direction perpendicular thereto.
The perfect realization of the in-plane switching completes enhancement of the viewing angle of the liquid crystal display device including halftone. Therefore, the liquid crystal having negative dielectric anisotropy is preferred as a liquid crystal used in the IPS system from the foregoing standpoint.
The IPS system employs an opaque metallic comb electrode in a stripe form provided on an inner surface of one of the pair of electrodes.
In recent years, a modified system of the IPS system has been proposed in that the comb electrode is formed with a transparent electroconductive substance, such as ITO (indium tin oxide), instead of the opaque metallic electrode, and is arranged at a shorter interval than the conventional IPS system, and a liquid crystal material having negative dielectric anisotropy, whereby the entire liquid crystal present above the transparent comb electrode can be subjected to orientation change only with an electric field formed at the periphery of the comb electrode, so as to improve the transmittance and the opening ratio.
Literatures relating to the foregoing proposal include Asia Display 1998, pp. 371-374, by S. H. Lee, S. L. Lee and H. Y. Kim and SID digest 1999, pp. 202-205, by S. H. Lee, H. Y. Kim and T. Y. Eom.
The foregoing literatures report that in the IPS system combining the liquid crystal material having negative dielectric anisotropy and the short interval transparent comb electrode, such transmittance that is close to the TN system can be realized with maintaining such wide viewing angle characteristics that is equivalent to the IPS system.
It has been known in a liquid crystal display device that in the case where a liquid crystal driving voltage waveform having a direct current voltage superposed is applied to a liquid crystal layer, the direct current voltage (direct current offset voltage) remains in the liquid crystal layer even when the direct current voltage is removed.
As discussed in S. Matsumoto, Ekishou Display Gijutu (Liquid Crystal Display Technique), published by Sangyo Tosho Co., Ltd., Chap. 2, pp. 70-73, the application of the driving voltage waveform having a direct current voltage superposed to the liquid crystal layer may occur in an active matrix liquid crystal display device in an ordinary liquid crystal operation due to the structure of the active driving element of the liquid crystal display device, and it is difficult to completely prevent the superposing phenomenon of a direct current voltage when gradation display is conducted. The phenomenon is common to both the TN system and the IPS system conventionally employed.
The remaining direct current voltage affects the brightness in liquid crystal display devices of both the TN system and the IPS system, and difference in brightness is caused between a part applied with the direct current voltage and a part not applied therewith or between parts having different intensities of the applied direct current voltage.
Therefore, in the case, for example, where texts or graphics are displayed under ordinary driving conditions for a long period of time, such a phenomenon occurs that the texts or graphics that have been displayed are displayed for a certain period after turning off the display.
As a result, uniformity of display is impaired. Such a phenomenon is called as an after image of a liquid crystal display device, which is gradually decreased in intensity with the lapse of time after formation thereof and is finally disappeared, but there are cases where a period of 30 minutes or more is required to disappear upon viewing with the naked eye.
As a mechanism that when a direct current voltage is applied, the direct current offset voltage remains in a liquid crystal layer, a model explaining by behavior of ions in the liquid crystal layer in the conventional TN system as an example has been proposed in Shingaku Gihou (Technical Research Report of Institute of Electronics, Information and Communication Engineers), EID96-89, pp. 29-34 (1997-01).
According to the model, a direct current voltage charged in an oriented film and absorption of ions on an orientation film for directing the liquid crystal are considered as factors of the direct current voltage remaining in the liquid crystal layer, and it sums up that the remanence of the direct current voltage for several minutes is caused by charging and relaxation of the orientation film, and the remanence of the direct current voltage for a longer period is caused by absorption of ions on the orientation film.
The IPS system suffers more frequent occurrence of the after image than the TN system. In the TN system, only liquid crystal orientation controlling layers and a liquid crystal layer are present between a pixel electrode and a counter electrode, and an electric field is applied to the pixel electrode, the liquid crystal orientation controlling layer, the liquid crystal layer, the liquid crystal orientation controlling layer and the counter electrode in this order.
On the other hand, the IPS system has an insulating layer in addition to the liquid crystal layer and the liquid crystal orientation controlling layers between the pixel electrode and the counter electrode, and the electric field is applied to the pixel electrode, the liquid crystal orientation controlling layer, the liquid crystal layer, the liquid crystal orientation controlling layer, the insulating layer and the counter electrode in this order.
That is, because charging and relaxation of the orientation films and the insulating film are considered while only the orientation film is considered for the remanence of the direct current voltage in the TN system, the after image is liable to occur in the IPS system as compared to the TN system.
In TN system, the after image is liable to occur when an insulating layer is arranged on the pixel electrode or the counter electrode to sandwich the insulating layer between pixel electrode and the counter electrode, to which the electric field is applied.
However, the occurrence of the after image can be suppressed by opening holes on the insulating film at positions above the pixel electrode and the counter electrode, so as to apply the electric field on the pixel electrode, the liquid crystal orientation controlling layer, the liquid crystal layer, the liquid crystal orientation controlling layer and the counter electrode in this order.
JP-A-7-159786 proposes a method for suppressing the remanence phenomenon of the direct current voltage caused by charging and relaxation of the orientation film by optimizing the dielectric constant and the specific resistance of the orientation film and the liquid crystal. In order to suppress the after image by accelerating the charging and relaxation of the orientation film and the insulating film, it is effective that the liquid crystal has a lower specific resistance.
The specific resistance of the liquid crystal can be decreased by adding a substance that decreases the specific resistance of the liquid crystal. For example, JP-A-11-302652 proposes that the specific resistance of a liquid crystal can be adjusted by adding an oxidative compound to the liquid crystal.
The after image causes no problem when a liquid crystal containing the oxidative compound is used in the IPS system using a liquid crystal material having positive dielectric anisotropy and the IPS system combining a liquid crystal material having positive dielectric anisotropy and a short interval transparent comb electrode.
However, the occurrence of the after image cannot be completely avoided by using the oxidative compound in the IPS system using a liquid crystal material having negative dielectric anisotropy and the IPS system combining a liquid crystal material having negative dielectric anisotropy and a short interval transparent comb electrode.
The oxidative compound has a molecular structure that is similar to the liquid crystal material having positive dielectric anisotropy. That is, one of the both ends in the long axis of the molecule is formed with a group having polarity other than a group having no polarity or extremely weak polarity, such as an alkyl group or an alkoxy group.
The other of the ends is formed with a group having high polarity, such as a cyano group or a fluorine-containing group, and it is polarized in the longer axis of the molecule rather than the shorter axis of the molecule.
The liquid crystal molecule having positive dielectric anisotropy is also polarized in the longer axis of the molecule rather than the shorter axis of the molecule. In other words, the liquid crystal material having positive dielectric anisotropy and the oxidative compound agree to each other in the molecular axis direction and the polarizing direction. It is therefore considered that the remaining direct current voltage can be effectively relaxed.
However, in the case of the liquid crystal material having negative dielectric anisotropy, the both ends in the longer axis direction of the molecule are formed with a group having no polarity or extremely weak polarity, such as an alkyl group or an alkoxy group, and one end in the shorter axis of the molecule is formed with a group having high polarity, such as a cyano group and a fluorine-containing group. Therefore, it is polarized in the shorter axis of the molecule rather than the longer axis of the molecule.
As described in the foregoing, the liquid crystal material having negative dielectric anisotropy does not agree to the oxidative compound, which has a molecular structure that is similar to the liquid crystal material having positive dielectric anisotropy, in the molecular axis direction and the polarizing direction. Therefore, it is considered that the remaining direct current voltage cannot be effectively relaxed.
The invention has been developed to solve the foregoing problems associated with the conventional art.
An object of the invention is to provide an active matrix liquid crystal display device of an IPS system that is difficult to cause a state of ununiform display remaining after application of a direct current voltage, i.e., an after image, in an IPS system using a liquid crystal material having negative dielectric anisotropy and an IPS system combining a liquid crystal material having negative dielectric anisotropy and a short interval transparent comb electrode.
Another object of the invention is to provide an active matrix liquid crystal display device of an IPS system that is difficult to cause an after image by adding a dissociative dopant and modifying the shape of the electrode even in the case where a liquid crystal material having positive dielectric anisotropy.
In order to accomplish the objects, the invention relates to an active matrix liquid crystal display device comprising
a pair of substrates;
a liquid crystal layer sandwiched by said pair of substrates;
orientation films defining an orientation direction of a liquid crystal molecule of said liquid crystal layer, said orientation films being arranged between said pair of substrates and said liquid crystal layer; and
a pixel electrode and a counter electrode applying a voltage to said liquid crystal layer,
said liquid crystal molecule of said liquid crystal layer having negative dielectric anisotropy, and said liquid crystal layer containing a dopant having a dissociative group.
A liquid crystal display device causing less after image can be provided.
In order to accomplish the objects, the invention relates to an active matrix liquid crystal display device comprising a pair of substrates, at least one of which is transparent; liquid crystal orientation controlling layers formed on surfaces of the pair of substrates facing each other; a liquid crystal layer comprising a liquid crystal composition having negative dielectric anisotropy arranged between the pair of substrate to make in contact with the liquid crystal orientation controlling layers (orientation films); a pixel electrode and a counter electrode formed on one of the pair of substrates through an insulating film; and an active element connected to the pixel electrode and the counter electrode, the liquid crystal layer containing a dopant having a dissociative group only in a shorter axis direction of the molecule and having an alkyl group or an alkoxy group on both ends of a molecular axis direction.
According to the liquid crystal display device, the liquid crystal material having negative dielectric anisotropy and the dopant having a dissociative group only in a shorter axis direction of the molecule agree to each other in the molecular axis direction and the polarizing direction.
Therefore, the remaining direct current voltage can be effectively relaxed, and a liquid crystal display device causing less after image can be provided.
The dopant having a dissociative group referred herein means an acidic dissociative substance or a basic dissociative substance, or in other words, a substance generating an H+ ion through dissociation by itself in a polar solvent or generating an OHxe2x88x92 ion through a reaction with water.
Specific examples thereof include a carboxylic acid (including an anhydride thereof), an amide, an amine and an alcohol. When these substances are added to a liquid crystal, the ion concentration in the liquid crystal is increased, so as to decrease the specific resistance.
It is preferred that the pixel electrode and the counter electrode are transparent electrode formed with a transparent electrode, such as ITO, and electric insulation between the pixel electrode and the counter electrode is maintained by a transparent insulating film. For example, the pixel electrode may be a short interval transparent comb electrode, and the counter electrode may be a solid electrode. The transparent insulating film may be constituted, for example, with IZO, silicon nitride, titanium oxide, silicon oxide and a mixture thereof.
When the dopant has the following structure represented by the general formula (I), it can effectively relax the remaining direct current voltage, so as to provide a liquid crystal display device exhibiting less after image. The dopant having the following structure has a molecular structure that is similar to a liquid crystal material having negative dielectric anisotropy.
That is, both ends in the longer axis direction of the molecule are formed with a group having polarity other than a group having no polarity or extremely weak polarity, such as an alkyl group or an alkoxy group.
Since it has a dissociative group in the shorter axis direction of the molecule, it is strongly polarized in the shorter axis direction of the molecule.
The liquid crystal molecule having negative dielectric anisotropy is also polarized in the shorter axis direction of the molecule rather than the longer axis direction of the molecule.
The liquid crystal material having negative dielectric anisotropy and the dopant having the following structure agree to each other in the molecular axis and the polarizing direction.
Accordingly, the remaining direct current voltage can be effectively relaxed. 
Wherein Y1 represents any one of xe2x80x94COGH, xe2x80x94CONH2xe2x80x94NH2, xe2x80x94OH, xe2x80x94NHR or NR2; Y2 represents any one of hydrogen, xe2x80x94F, xe2x80x94CN, xe2x80x94COOH, xe2x80x94CONH2, xe2x80x94NH2 or xe2x80x94OH; Y3 represents any one of hydrogen, xe2x80x94F, xe2x80x94CN, xe2x80x94COOH, xe2x80x94CONH2xe2x80x94NH2 or xe2x80x94OH; Y4 represents any one of hydrogen, xe2x80x94F, xe2x80x94CN, xe2x80x94COOH, xe2x80x94CONH2xe2x80x94NH2 or xe2x80x94OH; X1 represents any one of a single bond, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94COCH2xe2x80x94, xe2x80x94CH2xe2x80x94COxe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94 or xe2x80x94CHxe2x95x90CH; X2 represents any one of a single bond, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94COCH2xe2x80x94, xe2x80x94CH2xe2x80x94COxe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94 or xe2x80x94CHxe2x95x90CH; A1 represents any one of a single bond, a phenylene group or a cyclohexylene group; R1 represents any one of an alkyl group or an alkoxy group; and R2 represents any one of an alkyl group or an alkoxy group.
Furthermore, when the dopant has the following structure represented by the general formula (II), it can more effectively relax the remaining direct current voltage, so as to provide a liquid crystal display device exhibiting less after image. The dopant having the following structure has a molecular structure that is further similar to a liquid crystal material having negative dielectric anisotropy. That is, both ends in the longer axis direction of the molecule are formed with a group having polarity other than a group having no polarity or extremely weak polarity, such as an alkyl group or an alkoxy group.
In this application, single bond means direct connection. In case X1 is single bond, for example, A1 connect directly to benzene structure.
Since it has a highly dissociative group or a group having high polarity at one end of the shorter axis direction of the molecule, it is strongly polarized in the shorter axis direction of the molecule. Since the liquid crystal material having negative dielectric anisotropy has a group having high polarity, such as a cyano group or a fluorine-containing group, at one end of the shorter axis direction of the molecule, it is polarized in the shorter axis direction of the molecule rather than the longer axis direction of the molecule.
The liquid crystal material having negative dielectric anisotropy and the dopant having the following structure agree to each other in the molecular axis and the polarizing direction. Accordingly, the remaining direct current voltage can be effectively relaxed. 
Wherein Y1 represents any one of xe2x80x94COOH, xe2x80x94CONH2, xe2x80x94NH2, xe2x80x94OH, xe2x80x94NHR or NR2; Y2 represents any one of hydrogen, xe2x80x94F, xe2x80x94CN, xe2x80x94COOH, xe2x80x94CONH2, xe2x80x94NH2 or xe2x80x94OH; X1 represents any one of a single bond, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94COCH2xe2x80x94, xe2x80x94CH2xe2x80x94COxe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94 or xe2x80x94CHxe2x95x90CH; X2 represents any one of a single bond, xe2x80x94COxe2x80x94Oxe2x80x94, xe2x80x94Oxe2x80x94COxe2x80x94, xe2x80x94COCH2xe2x80x94, xe2x80x94CH2xe2x80x94COxe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94 or xe2x80x94CHxe2x95x90CH; A1 represents any one of a single bond, a phenylene group or a cyclohexylene group; A2 represents any one of a single bond, a phenylene group or a cyclohexylene group; R1 represents any one of an alkyl group or an alkoxy group; and R2 represents any one of an alkyl group or an alkoxy group.
The content of the dopant in the liquid crystal is generally 100 ppm (1xc3x9710xe2x88x924% by weight) or more, and preferably 1,000 ppm or more. When a non-liquid crystal substance is incorporated in a liquid crystal, the characteristics of the liquid crystal (liquid crystal property) is deteriorated, and when a too large amount of the non-liquid crystal substance is incorporated, such a temperature range in that the liquid crystal behaves as a liquid crystal nature becomes narrow. In the invention, since the dopant is incorporated in the liquid crystal in an amount of 100 ppm or more, preferably 1,000 ppm or more, the after image can be suppressed while decrease of the liquid crystal property of the liquid crystal is suppressed to the allowable range to constitute an active matrix liquid crystal display device, whereby a liquid crystal display device having excellent liquid crystal characteristics and less after image can be provided.
When the specific resistance of the liquid crystal is from 1.0xc3x97109 to 1.0xc3x971012 xcexa9xc2x7cm, a liquid crystal display device having less after image can be provided. When the liquid crystal has a specific resistance of more than 1.0xc3x971012 xcexa9xc2x7cm, the effect of suppressing the after image cannot be conspicuously obtained, and when the liquid crystal has a specific resistance of less than 1.0xc3x97109 xcexa9xc2x7cm, high display quality cannot be maintained.
The orientation film as the liquid crystal orientation controlling layer is formed to have a film thickness of from 20 nm to 300 nm. When the film thickness of the orientation film is less than 20 nm, the uniformity of the orientation film is deteriorated since the unevenness of the surface of the ITO film or the IZO film, which is formed under the orientation film, is from 10 nm to 20 nm, so as to cause display unevenness and to cause printing unevenness of the orientation film upon forming the orientation film. When the film thickness of the orientation film is more than 300 nm, the orientation film is ununiformly dried, which causes display unevenness.
The insulating film is formed to have a film thickness of from 0.1 xcexcm to 4 xcexcm. When the film thickness of the insulating film is less than 0.1 xcexcm, the insulating property of the film is deteriorated, and when it exceeds 4 xcexcm, the after image becomes conspicuous.
As the liquid crystal having negative dielectric anisotropy, a liquid crystal containing a liquid crystal molecule having a difluorinated benzene structure in the molecule and a liquid crystal containing a liquid crystal molecule having a dicyanobenzene structure in the molecule can be used.
Furthermore, a liquid crystal containing both a liquid crystal molecule having a difluorinated benzene structure in the molecule and a liquid crystal molecule having a dicyanobenzene structure in the molecule can also be used. A liquid crystal containing a liquid crystal molecule having a monocyanocyclohexane structure in the molecule can also be used as the liquid crystal having negative dielectric anisotropy.
A liquid crystal containing both a liquid crystal molecule having a difluorinated benzene structure in the molecule and a liquid crystal molecule having a monocyanocyclohexane structure in the molecule can also be used.
Moreover, in the case of a liquid crystal having positive dielectric anisotropy is used, the dissociative dopant can be added, and the structures of the pixel electrode and the counter electrode are normalized, whereby the occurrence of the after image can be suppressed.
One means is an active matrix liquid crystal display device comprising a pair of substrates; a liquid crystal layer sandwiched by said pair of substrates; orientation films defining an orientation direction of a liquid crystal molecule of said liquid crystal layer, said orientation films being arranged between said pair of substrates and said liquid crystal layer; and a pixel electrode and a counter electrode applying a voltage to said liquid crystal layer, said liquid crystal molecule of said liquid crystal layer having positive dielectric anisotropy, and said liquid crystal layer containing a dopant having a dissociative group.
A means with a liquid crystal composition comprising from 100 ppm to 1,000 ppm of a dopant having a dissociative group only in a shorter axis direction of a molecule and having an alkyl group or an alkoxy group on both ends of said shorter axis direction of a molecule is effective.